Magnetothermopower of nanocomposites in the vicinity of the percolation threshold
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Magnetothermopower of Nanocomposites in the Vicinity of the Percolation Threshold V. A. Belousova, A. B. Granovskiœb, Yu. E. Kalinina, and A. V. Sitnikova a
b
Voronezh State Technical University, Voronezh, 394026 Russia Moscow State University, Vorob’evy gory, Moscow, 119992 Russia e-mail: [email protected] Received May 28, 2007
Abstract—The field dependences of the thermopower of composites with Co and Co45Fe45Zr10 nanoparticles in the Al2On insulator matrix are studied in magnetic fields up to 10 kOe at room temperature with compositions up to the percolation threshold (i.e., in the region where tunnel conductivity takes place). In composites obtained in argon, negative magnetothermopower (i.e., a decrease in the thermopower in strong magnetic fields) is observed, while positive magnetothermopower is observed in composites obtained in the atmosphere of argon and oxygen. It is shown that the theory developed for tunnel magnetothermopower in nanocomposites makes it possible to explain the results on a qualitative level in the case when the local density of electron states at the surface of nanoparticles depends on the sputtering conditions. Nanocomposites CoFeZrx(Al2On)100 – x obtained in argon and nitrogen display a strong asymmetry of magnetothermopower relative to the magnetic field direction; this anisotropy is associated with anisotropy of these nanostructures. PACS numbers: 72.20.Pa, 72.80.Tm, 73.90.+f DOI: 10.1134/S1063776107120138
1. INTRODUCTION Various magnetic, electric, galvanomagnetic, optical, and magnetooptical properties of magnetic granular systems of the ferromagnetic metal-insulator type have been intensively studied in recent years. Systems containing nanosize grains are also referred to as magnetic nanocomposites. A distinguishing feature of magnetic nanocomposites with a metallic grain concentration close to the percolation threshold is the existence of tunnel-type transitions between metallic grains. Spin-dependent tunneling is one of basic mechanisms underlying various spintronics devices, both implemented and being developed [1]. Tunnel magnetoresistance is the clearest manifestation of spin-dependent tunneling both in multilayers and in nanocomposites [2]. Spin-dependent tunneling at high frequencies leads to the magnetorefractive effect [3] and is responsible for some peculiar magnetooptical properties [4]. In this paper, at attempt is made at experimental investigation of manifestations of spin-dependent tunneling in magnetothermopower of magnetic nanocomposites. A considerable change in thermopower S in a magnetic field H for nanocomposites was observed for the first time for Co46Al29O35 samples obtained by sputtering of a CoxAl1 – x target in an Ar + O2 atmosphere and later for Fe–Al–O) nanocomposites [5, 6]. Magnetothermopower was observed only for two compositions in the immediate vicinity of the percolation threshold.
The field dependence S(H) was described by the relation S ( H )/T = a + bρ ( 0 )/ρ ( H ),
(1)
where a and b are independent of the field an
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